Magnetic angular position sensor

ABSTRACT

A magnetic angular position sensor system is described herein. According to one exemplary embodiment the angular position sensor system comprises a shaft rotatable around a rotation axis, wherein the shaft has a soft magnetic shaft end portion. The system further comprises a sensor chip spaced apart from the shaft end portion in an axial direction and defining a sensor plane, which is substantially perpendicular to the rotation axis. At least four magnetic field sensor elements are integrated in the sensor chip, wherein two of the magnetic field sensor elements are spaced apart from each other and are only sensitive to magnetic field components in a first direction and wherein two of the magnetic field sensor elements are spaced apart from each other and are only sensitive to magnetic field components in a second direction, whereby the first and the second direction are mutually non-parallel and the first and the second direction being perpendicular to the rotation axis. Moreover, the system comprises a magnetic field source that magnetizes the shaft end portion, wherein the shaft end portion is formed such that a magnetic field in the sensor plane, which is caused by the magnetic field source, is rotationally symmetric with order N, wherein N is a finite integer number ≧1. The system also comprises circuitry that is coupled to the at least four magnetic field sensor elements and configured to calculate an angular position of the shaft by combining output signals of the at least four magnetic field sensor elements.

TECHNICAL FIELD

The invention relates in general to angular position sensors which makeuse of magnetic field sensors to measure an angle value, and moreparticularly to on-axis angular position sensors, systems and methodshaving a variety of applications, including in brushless direct current(DC) motors.

BACKGROUND

Magnetic field sensors can be used to sense an angle position of a shaftor other object. For example, a permanent magnet can be mounted on theshaft, and a magnetic field sensor can be arranged proximate to themagnet in order to sense a magnetic field generated by the magnet as itrotates with the shaft. When the magnetic field sensor is mounted nextto the shaft at a particular distance from the axis of rotation of theshaft, the sensor is often referred to as an “off-axis” magnetic angularposition sensor. Off-axis magnetic angular position sensors are oftenimplemented when the front side of the shaft is inaccessible (e.g. dueto a specific use or assembly of the shaft), and therefore sensorelements cannot be mounted on the axis of rotation. Conversely, an“on-axis” magnetic angular position sensor is mounted at or proximate toone end of the shaft facing its front side and, generally, in-line withor symmetrically to the axis of rotation. In some embodiments, on-axismagnetic field angle sensors can be designed to measure a magnetic fieldgradient. For this purpose, the magnetic field may be measured at twodifferent points, which are arranged at opposing sides of andsymmetrically to the axis of rotation. The gradient may then bedetermined in sufficient approximation for many applications bycombining the two measured magnetic field values, e.g. by subtraction.

In many applications it is a general design goal for magnetic angularposition sensors to be inexpensive while also being robust with respectto external magnetic fields and other disturbances and insensitive toassembly tolerances. One particular application for magnetic field anglesensors is in brushless DC (BLDC) motors for the detection of the(angular) shaft position during rotation. BLDC motors present achallenging environment for magnetic field sensors as they typicallyinclude strong rotating magnets and copper windings carrying largecurrents, both of which produce time-varying magnetic fields whichinterfere with the magnetic field used to measure angular position andthus result in a measurement error. These disturbing magnetic fields arestrongly inhomogeneous, which makes it difficult to eliminate theireffect on the angular position measurement. In view of these problemsthere is a general need for improvement in magnetic angular positionsensors.

SUMMARY

A magnetic angular position sensor system is described herein. Accordingto one exemplary embodiment the angular position sensor system comprisesa shaft rotatable around a rotation axis, wherein the shaft has a softmagnetic shaft end portion. The system further comprises a sensor chipspaced apart from the shaft end portion in an axial direction anddefining a sensor plane, which is substantially perpendicular to therotation axis. At least four magnetic field sensor elements areintegrated in the sensor chip, wherein two of the magnetic field sensorelements are spaced apart from each other and are only sensitive tomagnetic field components in a first direction and wherein two of themagnetic field sensor elements are spaced apart from each other and areonly sensitive to magnetic field components in a second direction,whereby the first and the second directions are mutually non-paralleland the first and the second directions are perpendicular to therotation axis. Moreover, the system comprises a magnetic field sourcethat magnetizes the shaft end portion, wherein the shaft end portion isformed such that a magnetic field in the sensor plane, which is causedby the magnetic field source, is rotationally symmetric with order N,wherein N is a finite integer number ≧1. The system also comprisescircuitry that is coupled to the at least four magnetic field sensorelements and configured to calculate an angular position of the shaft bycombining output signals of the at least four magnetic field sensorelements.

In accordance with another exemplary embodiment, the angular positionsensor system comprises a shaft rotatable around a rotation axis,wherein the shaft has a soft magnetic shaft end portion. The systemfurther comprises a sensor chip spaced apart from the shaft end portionin an axial direction and defining a sensor plane which is substantiallyperpendicular to the rotation axis. At least four magnetic field sensorelements are integrated in the sensor chip, wherein a first and a secondmagnetic field sensor element of the magnetic field sensor elements arespaced apart from each other and are sensitive to magnetic fieldcomponents in a first direction. A third and a fourth magnetic fieldsensor element of the magnetic field sensor elements are spaced apartfrom each other and are sensitive to magnetic field components in asecond direction, wherein the first and the second directions aremutually non-parallel and perpendicular to the rotation axis. Moreover,the system comprises a magnetic field source that magnetizes the shaftend portion, wherein the shaft end portion is formed such that amagnetic field in the sensor plane, which is caused by the magneticfield source, is rotationally symmetric with order N, whereby N is afinite integer number ≧1. The system also comprises a signal processingcircuit which is coupled to the at least four magnetic field sensorelements and configured to: calculate a first signal representing thedifference of the magnetic field components sensed by the first and thesecond magnetic field sensor elements; calculate a second signalrepresenting the difference of the magnetic field components sensed bythe third and the fourth magnetic field sensor elements; and calculatean angular position of the shaft by combining at least the first and thesecond signal.

Additionally, an electric motor assembly is described herein. Inaccordance with one exemplary embodiment the electric motor comprises astator including at least one stator coil, a rotor including at least ashaft, which has a front side and a soft-magnetic shaft end portion, anda printed circuit board (PCB), which is arranged such that it faces thefront side of the shaft. At least one sensor chip is attached to the PCBand spaced apart from the shaft end portion. At least four magneticfield sensor elements are arranged in the at least one sensor chip,wherein two of the magnetic field sensor elements are spaced apart fromeach other and are only sensitive to magnetic field components in afirst direction and two of the magnetic field sensor elements are spacedapart from each other and are only sensitive to magnetic fieldcomponents in a second direction, whereby the first and the seconddirection are substantially mutually non-parallel and the first and thesecond directions are perpendicular to the rotation axis. A magneticfield source is provided which magnetizes the shaft end portion, whereinthe shaft end portion is formed such that the magnetic field componentsin the first and the second direction are rotationally symmetric withorder N and N is a finite integer number ≧1. An evaluation circuit iscoupled to the at least four magnetic field sensor elements andconfigured to calculate an angular position of the shaft by combining atleast four output signals of the at least four magnetic field sensorelements. Moreover, power electronic circuitry is arranged on the PCBand coupled to the stator coils and configured to supply an operatingcurrent to the stator coils.

Furthermore, a method for measuring an angular position of a shaft whichincludes a soft magnetic shaft end portion is described herein. Inaccordance with one exemplary embodiment, the method includesmagnetizing the shaft end portion, wherein the shaft end portion isformed such that the magnetic field components in a first and a seconddirection in a sensor plane, which is substantially perpendicular to arotation axis of the shaft, are rotationally symmetric with order N,whereby N is a finite integer number ≧1. The method further includessensing magnetic field components in the first direction at at least afirst and a second location in the sensor plane, wherein the secondlocation is different from the first location, as well as sensingmagnetic field components in the second direction at at least a thirdand a fourth location in the sensor plane, wherein the fourth locationis different from the third location. Moreover, the method includescalculating an angular position of the shaft with respect to itsrotation axis based on the difference of the magnetic field componentsat the first and the second location and on the difference of themagnetic field components at the third and the fourth location.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the followingdescription and drawings. The components in the figures are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention. Moreover, in the figures, likereference numerals designate corresponding parts. In the drawings:

FIG. 1 is a sectional view illustrating a longitudinal section of anon-axis angular position sensor.

FIG. 2 illustrates disturbing magnetic field due to high currents on thePCB.

FIG. 3 illustrates a longitudinal section of an on-axis angular positionsensor arrangement in accordance with one embodiment and a cross-sectionthrough a shaft rotating about the axis.

FIG. 4 illustrates three different examples of shaft end-portions to beused in an on-axis angular position sensor.

FIG. 5 illustrates another example of a shaft end-portion to be used inan on-axis angular position sensor.

FIG. 6 illustrates a longitudinal section of an on-axis angular positionsensor arrangement in accordance with another embodiment, wherein thegroove in the shaft end-portion is filled with a permanent magnet.

FIG. 7 illustrates an example of a shaft end-portion which has arotational symmetry of order 3.

FIG. 8 illustrates a longitudinal section through a further embodimentof an angular position sensor arrangement with a flexible spring ofmagnetic material.

FIG. 9 illustrates—in a top view on the circuit board carrying amagnetic field sensor—two exemplary implementations of the flexiblespring shown in FIG. 7.

FIG. 10 illustrates a longitudinal section through a further embodimentof an angular position sensor arrangement with a flexible spring ofmagnetic material.

FIG. 11 illustrates a longitudinal section through a further embodimentof an angular position sensor arrangement with a flexible spring ofmagnetic material.

FIGS. 12-14 illustrate different exemplary arrangements of magneticfield sensor elements to be used in a magnetic angular position sensor.

FIGS. 15-16 illustrate two different examples of how an on-axis angularposition sensor can be used in a brushless DC (BLCD) motor.

DETAILED DESCRIPTION

The embodiments described herein relate to magnetic field sensors usedin magnetic angular position sensors with on-axis arrangements of sensorelements relative to a rotation axis of a magnet or a shaft. In oneembodiment, a magnetic angular position sensor is mounted in an on-axisconfiguration relative to a rotatable shaft. The shaft comprises an endportion that may be composed of a soft magnetic material or a permanentmagnet. The end portion of the shaft may have a front surface facing thesensor and be rotationally asymmetric with respect to a rotation axis ofthe shaft. The sensor comprises at least three magnetic field sensorelements arranged in a plane generally perpendicular to the rotationaxis. Circuitry coupled to the magnetic field sensor elements isconfigured to estimate a rotational position of the shaft by combiningthe signals of the at least three magnetic field sensor elements. Whilenumerous applications exist for various embodiments of the sensor, someembodiments may, among other applications, be particularly suitable foruse in or with BLDC motors. Any particular references to directions madeherein (i.e., downward, upward, right, left, etc.) are merely used forreference to a specific drawing and are not are limiting with regard tothe claims.

An example of an on-axis angular position sensor system 100 isillustrated in FIG. 1. The sensor system 100 includes a magnetic fieldsource, such as a permanent magnet 102, and a sensor package 106, whichis arranged on a printed circuit board (PCB) 108. The sensor package 106is—at least—partially arranged between the permanent magnet 102 and anend portion of a shaft 104, which is configured to rotate around arotation axis. In FIG. 1, the rotation axis is defined as the z-axis ofa Cartesian coordinate system, and the x-y-plane is perpendicular to therotation axis.

In some embodiments, the permanent magnet 102 comprises a ferritemagnet. Ferrite magnets can be less expensive than other types ofmagnets (e.g., rare-earth magnets) and contribute to lower overallsystem costs. However, in other embodiments rare earth magnets or othertypes of magnets can be used. In some embodiments, the magnet 102 caneven be omitted. In this case the end portion of the shaft 104 has asignificant remanent magnetization. For example, the magnet 102 maycomprise an Sr-ferrite, a Ba-ferrite, or some other ferrite, AlNiCo, arare-earth material such as NdFeB or SmCo, or some other suitablematerial. Generally, the magnet 102 comprises a material having aremanent magnetization of at least about 100 mT and sufficientcoercivity to ensure stability of magnet 102.

In the example depicted in FIG. 1, the magnet 102 is magnetized in anaxial direction (see arrows in FIG. 1). That is, the orientation ofmagnetization of the magnet 102 is generally parallel to the rotationaxis of shaft 104 (z-axis). In other embodiments, the magnet 102 may bemagnetized in a radial direction. However, the magnet 102 is usually(but not necessarily) rotationally symmetric (i.e., has both geometricand magnetic symmetry such that the geometry and magnetization do notdepend on the angular position relative to the rotation axis). Forexample, the magnet 102 may have a cylindrical shape as shown in theembodiment of FIG. 1. However, other shapes and directions ofmagnetization may be used in other embodiments. In general, however, themagnet 102 has rotational symmetry and, during operation, provides amagnetic field which is rotationally symmetric with regard to therotation axis. The non-rotationally symmetric geometry of the endportion of the shaft 104, however, interrupts this rotational symmetry.Due to the resulting asymmetry, the angular position of the shaft 104can be detected by the angular position sensor.

As shown in FIG. 1, the shaft 104 comprises an end portion with a frontside (surface 105), which is arranged opposite to the sensor package106. As mentioned, the shaft 104 is generally arranged rotationallysymmetric, whereas the front side 105 of the shaft 104 may beasymmetrical with respect to the rotation axis. In the example of FIG.1, the front side 105 is tilted by an angle α with respect to thexy-plane. In one embodiment, the angle α is about 15 degrees, though itcan be larger or smaller in other embodiments. Theoretically the angle αcan be between 0 degrees and 90 degrees, in practical implementations αcan be, for example, between about 5 degrees and about 25 degrees. Dueto the fact that a is greater than 0 degrees, the size of the air gap(in the z-direction) generally depends, at a specific position in thexy-plane, on the angular position of the shaft 104.

In some embodiments, the shaft 104 comprises ferrous material, such as asoft magnetic material, like iron or mild steel, with a relativepermeability μ_(R) in the range of about 1,600 (steel) to 4,000 (iron).Generally, shaft 104 may comprise a ferrous material with a relativepermeability greater than about 100 or greater than about 1,000. In someembodiments, only an end portion of the shaft comprises magneticmaterial, whereas the rest of the shaft 104 is mainly made of anon-magnetic material such as a non-iron alloy, a non-magnetic orlow-magnetic steel, or other materials.

As shown in FIG. 1, the sensor package 106 is generally arranged betweenthe permanent magnet 102 and the front side 105 of the shaft 104. Thexy-plane is defined as the plane in which the magnetic field sensorelements 114 a, 114 b are arranged within the chip package. Thereforethe axial distance between the xy-plane and the front side 105 of theshaft 104 is the effective air gap of the magnetic circuit. The sensorpackage 106 may be arranged substantially in-line with the rotation axisand, thus, forms an on-axis sensor arrangement. The sensor package 106includes at least one semiconductor die 110 in which the magnetic fieldsensor elements 114 a, 114 b are integrated. The semiconductor die 110is generally perpendicular to the rotation axis (z-axis) and comprises aprimary surface 112, which is the sensitive plane (i.e. the xy-plane) ofthe sensor elements 11 a and 114 b. Only two sensor elements 114 a and114 b are visible in the cross-sectional view of FIG. 1. However, theangular position sensor includes at least three sensor elements, in someconfigurations at least four sensor elements. In both cases, the sensorelements are mutually spaced apart from each other, and each sensorelement provides a separate sensor signal that represents the magneticfield component at the location of the respective sensor element and inthe sensitive direction of the respective sensor element.

In some embodiments, the package 106 is a surface-mounted device (SMD),in which the semiconductor die 110 is attached to a leadframe 116. Theleadframe 116 comprises pins, which are soldered to the PCB 108. In theembodiment of FIG. 1, the permanent magnet 102 is arranged at leastpartially within an opening 130 in the PCB 108, such that permanentmagnet 102 can touch a backside of the package 106. However, the openingis not needed in other embodiments in which the permanent magnet 102 canbe mounted under the PCB 108. For example, in one embodiment, thepermanent magnet 102 is mounted to the backside of PCB 108. In thiscontext the backside of the package is the side which is opposite to thefront side, and the front side of the package 106 is the side whichfaces the front side 105 of the shaft. It should be noted that theset-up of the angular position sensor may be different from the exampleshown in FIG. 1 in other embodiments. For example, a bare die could bemounted to a PCB instead of using a chip package. In other embodiments,the sensor elements 114 a and 114 b may be arranged in separatesemiconductor dies in one or more chip packages. In some embodiments thepermanent magnet 102 may be attached to the bottom side of the PCB 108or integrated in the sensor package 106.

The sensor elements 114 (collectively for 114 a and 114 b) may either besensitive to magnetic field components in an axial direction or tomagnetic field components in a radial direction. In various embodiments,the sensor elements 114 may comprise magneto-resistive (MR) sensorelements (e.g., AMR, GMR, TMR, CMR and others), giant magneto-impedance(GMI) sensor elements, Hall-effect sensor elements (e.g., vertical Hallsensor elements), MAGFETs, and other magnetic field sensor elements andcombinations thereof which are suitable to sense magnetic fieldcomponents in a plane that is perpendicular to the rotation axis of theshaft whose angular position is to be measured. In various embodiments,the sensor elements 114 are oriented such that they are either sensitiveto a magnetic field component in an x-direction or in an y-direction,wherein the rotation axis of the shaft is defined as extending along az-direction. the x-, y- and z-directions form a Cartesian coordinatesystem.

Generally, the sensor set-ups described herein can be used in brushlessDC (BLDC) motors. Such BLDC motors employ permanent magnets to magnetizethe rotor (armature) of the BLDC motor, whereas stator windings are usedto form coils to magnetize the stator of the BLDC motor. Current pulsesare applied to the stator coils, wherein the current pattern is designedto achieve a desired torque and/or rotation of the rotor. To allow acompact design of the BLDC motor, on-axis angular position sensors areused, wherein the printed circuit board (PCB) carrying the powerelectronics generating the mentioned current pattern usually alsocarries the components of the angular position sensor. Exemplaryembodiments showing the application of an on-axis angular positionsensor in BLDC motors are shown later in FIGS. 15 and 16.

FIG. 2 illustrates how disturbing magnetic fields can affect the fieldmeasurement by the magnetic field sensor elements 114. In the example ofFIG. 2 only the sensor package 106 is shown together with acurrent-carrying element 201 (e.g. a strip line or a power semiconductordevice) mounted on the PCB 108. The current i₀ passes through element201 substantially parallel to the y-axis, thus generating magnetic fieldH₀ illustrated by the magnetic field lines in FIG. 1. As can be seenfrom the field lines, the magnetic field H₀ comprises a significantcomponent H_(0,z) in z-direction at the location of the magnetic fieldsensor elements which is highly inhomogeneous. This magnetic fieldcomponent H_(0,z) may superpose with the magnetic field of the permanentmagnet 102 (not shown in FIG. 2) and cause a measurement error whendetermining the angular position from the sensor signals. In practice,the magnetic field components H_(0,x) and H_(0,y) perpendicular to thez-axis (in the xy-plane) have a significantly smaller magnitude at thelocation of the magnetic field sensor elements. To minimize the effectof disturbing magnetic fields, which are generated by currents generatedby power electronic devices on the PCB 108, magnetic field sensorelements 114 are used which are only sensitive to magnetic fieldcomponents in the x- or y-direction and are substantially insensitive tomagnetic field components in the z-direction (axial direction).Moreover, pairs of sensor elements are used to implement a differentialmeasurement as will be explained further below (see FIGS. 11-13). Thedifferential measurement is used to implement a kind of gradiometer,which senses the spatial gradient of the measured magnetic fieldcomponents and thus eliminates the effect of homogeneous disturbingmagnetic fields (generated externally of the PCB 108 or the BLDC motor).Combining the use of magnetic field sensors which are substantiallyinsensitive in an axial (i.e. z-) direction with a differentialmeasurement principle allows for an angular position measurement whichis robust against disturbing magnetic fields generated on board (i.e. bythe power electronics on the PCB 108) as well as off board (i.e.substantially homogeneous external magnetic fields).

In the embodiments described herein the end portion of the shaft 104 hasa rotational symmetry of a specific order, wherein rotational symmetryof order N (also called N-fold rotational symmetry) means that eachrotation by an angle of 360°/N does not change the object. N is a finite(non-zero and non-infinite) integer number equal to or greater than 1.It is noted that a symmetry of order 1 is actually not a symmetry, asonly a full rotation by 360° of the shaft yields an unchanged geometry.In contrast, a symmetry of order 2 means that a rotation of 180° of theshaft yields an unchanged geometry. Similarly, a symmetry of order 3means that a rotation of 120° of the shaft yields an unchanged geometry.In the example of FIG. 3, the end portion of the shaft 104 has a recessP (groove) in its front side. The recess P has the shape of a slitrunning straight (e.g. along the y-direction) through the rotation axis,which results in a shaft end portion having a symmetry of order 2 (seesection A-A′ in FIG. 3). Except for the end portion of the shaft 104,the set-up of the angular position sensor arrangement shown in FIG. 3 isalmost identical with the example of FIG. 1. The permanent magnet 102,however, is mounted on the back-side of the printed circuit board (PCB)108 instead of being arranged at the back-side of the sensor package 106through an opening in the PCB 108 as shown in FIG. 1. The sensor package106 is mounted on the front side of the PCB 108 and includes at leastone semiconductor chip 110, which has several magnetic field sensorelements 114 a, 114 b integrated in the chip 110. All magnetic fieldsensor elements are sensitive to a magnetic field component in thexy-plane and insensitive to a magnetic field component in thez-direction (i.e. axial direction).

In an alternative embodiment, which is illustrated by the curved dashedline in FIG. 3 (and also in FIG. 6 discussed further below), the grooveP has the shape of a half-cylinder (half-circular cross section) insteadof a rectangular cross-section. Both embodiments have been simulated. Inboth embodiments the sensor chip 110 is arranged in the xy-plane at z=0,and the sensitive magnetic field sensor elements are at z=0.1 mm. Therotation axis is defined as x=y=0. In the first embodiment, thepermanent magnet has a diameter of 10 mm, is arranged coaxially to theshaft 104, and has an axial length of 5 mm (from z=−6.5 mm to z=−1.5mm). The remanent magnetization of the permanent magnet 102 is 1 Tesla;its relative permeability μ_(R) is 1.1. The diameter of the shaft 104 is6 mm, and the front side of the shaft 104 is at z=1 mm. The air gapbetween shaft and permanent magnet is thus 2.5 mm. The relativepermeability μ_(R) of the shaft 104 is 1700. The width w of the grooveis 2 mm and its depth is 3 mm (rectangular cross-section). With the dataabove, the magnetic field components H_(X), H_(Y) within the xy-plane(i.e. in the sensor plane, in which the sensor elements extend) can beapproximated (using linear regression in the simulated magnetic fielddata) as H_(X)=a·x/μ₀ with a=56.6 T/m and H_(Y)=b·x/μ₀ with b=10 T/m,wherein μ₀ is the vacuum permeability. In the second embodiment, thediameter of the permanent magnet is 4 mm, its remanent magnetization is1 T (magnetized parallel to the z-axis), and its axial length is 4 mm(from z=−4.2 mm to −0.2 mm). The shaft has a diameter of 6 mm and thegroove P is a half-cylinder with radio of 1.5 mm. The front side of theshaft is at z=1.8 mm so the air gap is 2 mm. Different to the firstembodiment, the groove P is filled with another permanent magnet 102′(like in the example of FIG. 6) having a magnetization of −1 T(magnetized parallel to the z-axis). The magnetic field sensor elementsare at z=0.5 mm. In this arrangement, the magnetic field components inthe x- and y-direction of the two permanent magnets constructivelysuperpose. The simulation yields an approximation of H_(X)=a·x/μ₀ witha=175.4 T/m and H_(Y)=b·x/μ₀ with b=102.2 T/m. As such, the secondmagnet 102′ makes the diametrical magnetic field components (x-,y-components) stronger and the axial magnetic field component(z-component) smaller. In an ideal implementation the two magnets shouldbe balanced to make axial magnetic field on the sensor elements vanish,because then small tilts of the sensor plane (xy-plane) with respect tothe rotation axis—which are inevitable due to assembly tolerances—havethe smallest possible effect on the accuracy of angular positionmeasurement.

As mentioned above, the groove P may be filled with a second permanentmagnet 102′. Generally, one permanent magnet is attached to the shaft104 (e.g. in the groove P) so that it rotates synchronously with theshaft 104 and the other permanent magnet (cf. permanent magnet 102 inFIG. 3) one is attached to the sensor package 106 (cf. FIG. 1) or thePCB 108 (cf. FIG. 3) so that it does not rotate. Both permanent magnetsare magnetized in a direction parallel to the rotation axis(z-direction), wherein one of the permanent magnets is magnetized inpositive z-direction, the other in the negative z-direction. This waytheir magnetic field contributions H_(X), H_(Y) in the xy-plane add up.One of the two permanent magnets may be omitted and the sensorarrangement would still be functional. Whether one or two magnets areused may depend on available space, costs, stray field robustness of thesurrounding electronic components, etc.

Different magnetic materials may be used for the two permanent magnets.The rotating permanent magnet can be made of, for example, injectionmolded magnet material, in which magnetic grains are embedded in apolymer matrix, whereas the non-rotating permanent magnet (permanentmagnet 102) may be a sintered magnet which cannot be easily manufacturedfor complex geometries. However, sintered magnets can have a very highremanent magnetization (in case of NdFeB material the remanence canexceed 1 T). In contrast thereto, plastic bonded or injection moldedmagnets have a lower remanent magnetization (in the case of NdFeBplastic bonded magnets the remanent magnetization may be up to only 0.6T).

In one specific embodiment the sensor chip 110, in which the magneticfield sensor elements are integrated, has a size of about 0.5 mm to 4mm, e.g. 1.5 mm, in the x-direction. Therefore the spacing of themagnetic field sensor elements is also in this range (see also FIGS.11-13). The width w (along the x-direction) of the groove P in theend-portion of the shaft 104 may have a similar size or be slightlylarger than the chip 110, e.g. 2 mm. The shaft diameter may range fromapproximately twice the width of the groove P (e.g. 4 mm)to—theoretically—arbitrarily large diameters. The depth d of the groove(i.e. the axial length along the z-direction) may exceed half of thewidth of the groove (i.e. more than 1 mm) and be e.g. the same as itswidth. In the latter case, the cross section (perpendicular to they-axis in the example of FIG. 3) of the groove is quadratic. Deepergrooves are possible. However, deeper grooves do not result in amagnetic behavior that is significantly different from the magneticbehavior of grooves having a depth that is equal to their width. FIG. 4illustrates three other examples of shaft end-portions, which havedifferent grooves P. In all three examples (FIGS. 4a, 4b, and 4c ) thegroove extends straightly through the rotation axis of the shaftend-portion. The example in FIG. 4a has a groove P with a V-shaped crosssection, the example of FIG. 4b has a grove P with a trapezoidal crosssection, wherein the cross section becomes narrower with increasingdepth, and the example of FIG. 4c has a grove P with a trapezoidal crosssection, wherein the cross section becomes wider with increasing depth.

Moreover, the groove P does not necessarily have a constant depth d. Thedepth d of the groove P may be greater towards the rotation axis andlower towards the perimeter of the shaft. Also the front-face (thesurface of the front side) of the end-portion is not necessarily a flatplane. In the example of FIG. 5 the front-face has a curvature. Forexample, the front surface may be part of a spherical surfaces, whosespherical center lies on the rotation axis. Alternatively, the frontsurface may be part of a cylindrical surface whose cylinder axis mayintersect the rotation axis at 90°. Such curvatures may help to “design”the magnetic field in a way that the signal level of the output signalsof the magnetic field sensor elements is maximized (or, in case of adifferential measurement, that the difference of two respective outputsignals of a pair of magnetic field sensor elements is maximized). FIG.5 illustrates two different longitudinal sections (section A-A′ andsection B-B′) and a bottom view of the same shaft end portion.

Generally, a shaft end-portion with N-fold rotational symmetry allows tomeasure an absolute angular position within the interval [0, 360°/N].The shaft 104 is that component of the sensor arrangement whichgenerally is machined most accurately as compared with other components.In contrast thereto, the permanent magnet 102 may be machinedsignificantly less accurately than the shaft 104. Neither the shape ofthe permanent magnet 102, nor its material homogeneity, nor itsmagnetization (with regard to magnitude, direction, and homogeneity),and its stability over time and temperature are well defined. However,in the embodiments described herein the permanent magnet 102 is usedonly to magnetize the shaft end-portion and does not rotate. In otherwords, the permanent magnet 102 does not define the angular position ofthe magnetic field; it only biases the shaft end-portion, which definesthe angular position of the magnetic field. Therefore small inaccuracies(with regard to geometry and magnetization) of the permanent magnet 102do not have a significant impact on the accuracy of the angular positionmeasurement. As, for the reasons mentioned above, the permanent magnet102 does not need to have a precise and complex shape and magnetization,a cost-effective sintered magnet may be used. Sintered rare-earthmagnets may have a high remanent magnetization of more than 1 T, whichresults in comparably strong magnetic fields through the sensor elementsand therefore increases robustness against noise and interference (i.e.the magnetic signal-to-noise ratio is high). Moreover, since thepermanent magnet 102 does not rotate, it does not produce large rotatingmagnetic fields which could disturb other electronic components. Theshaft end-portion may be arranged very closely to the sensor chip 110and thus to magnetic field sensor elements (e.g., 1 mm air gap spacingor even less, depending on the axial tolerance of the shaft). Thus thedistance between relevant magnetized portions of the shaft end-portionand sensor elements can be comparably small, which also increases themagnitude of the measured magnetic field components.

In the embodiments described above, the shaft end portion has arotational symmetry of order 2 (two-fold rotational symmetry) whichallows for an unambiguous angular position measurement in the rangebetween 0 and 180°. Due to the symmetry, the sensor arrangement cannotdistinguish between an angle φ and an angle φ+180°. However, anunambiguous angular position measurement within the full range from 0°to 360° is not necessary in many applications such as in brushless DC(BLDC) motors.

However, if an unambiguous angular position measurement is desiredthroughout the full range between 0° and 360°, the orientation of theremanent magnetization of the rotating permanent magnet 102′ may betilted with respect to the rotation axis (z-axis) as shown in theexample of FIG. 6. This means that—in addition to its magnetization inz-direction (i.e. parallel to the rotation axis) the rotatable magnetshould also have a certain (preferably small) amount of magnetizationperpendicular to the z-direction (i.e. along x- or y-directions or anyother diametrical direction). In FIG. 6 the dashed-line arrow inpermanent magnet 102′ indicates the previously explained remanentmagnetization antiparallel to the z-direction, whereas the solid-linearrow indicates the above-mentioned tilted magnetization.

In accordance with one embodiment, the rotating permanent magnet has,e.g., 80% of its magnetization along the z-direction and 20% along thex-direction (of any other diametrical direction like, e.g. they-direction), which entails a tilt of 14° with respect to the rotationaxis and would contribute a small magnetic field H_(X), which isessentially homogeneous throughout a sensor chip. The sensor system canthus discriminate between an angle φ and an angle φ+180° by evaluatingthe sum and the difference of the sensor signals of different magneticfield sensor elements (see FIG. 6, magnetic field sensor elements 114 aand 114 b), which are spaced apart within the sensor plane (xy-plane)and sensitive to magnetic field components orthogonal to the rotationaxis and insensitive to axial magnetic field components. Thedifferential signal (e.g. H_(X)(x=x₁,y=0)−H_(X)(x=−x₁,y=0)) obtainedfrom a pair of magnetic field sensor elements may be used to determinean exact angular position which is either in the range from 0 to 180° orin the range from 180° to 360°. This differential signal is a result ofthe magnetic effect of the groove and from axial magnetization of bothmagnets. The sum signal (e.g. H_(X)(x=x₁,y=0)+H_(X)(x=−x₁,y=0)) obtainedfrom the pair of magnetic field sensor element can be used to determinewhich of the two ranges the angle φ is in (i.e. whether or not to add180° to the previously determined angel φ). This sum signal is a resultof the diametrical magnetization of the rotatable magnet. Except for theshape of the groove P and the further permanent magnet 102′ arranged inthe groove, the example of FIG. 6 is identical with the example of FIG.3.

In one further embodiment, the remanent magnetization of the rotatingpermanent magnet 102′ is oriented parallel to the x-axis (or any otherdiametrical direction, e.g. the y-direction), which entails a tilt(which was 14° in the previous example) of 90°. However, thenon-rotating permanent magnet 102 is still magnetized parallel to thez-axis (and thus produces a rotation symmetric magnetic field). Theremanent magnetization of the permanent magnet 102′ should be strongenough that the resulting homogeneous diametrical field component isstronger than any potentially disturbing external magnetic fieldcomponent in this direction. As the diametrical field component is onlyused to distinguish the 0°-180° sector from the 180°-360° sector, thereare no specific requirements as to the precision of the magnetization.

In another example, the shaft end-portion is shaped rotationallyasymmetric, that is it is shaped to have a rotational symmetry of order1 so that measurement over the full range from 0° to 360° is possibleeven if the permanent magnets are magnetized only along the z-direction.Using this approach, the groove P could be shifted from the centertowards the perimeter of the shaft end portion. Alternatively, a shaftend-portion as shown in FIG. 1 could be used. Moreover, the depth of thegroove P could vary in a rotationally asymmetric manner.

In the example of FIG. 7 the shaft end-portion is shaped to have arotation symmetry of order 3. That is, an unambiguous angular positionmeasurement is possible within the sector from 0° to 120° (360°/3). Ascan be seen in the bottom view shown in FIG. 7, the groove P splits upat the center of the shaft into two branches so as to form a Y-shape.Generally, the shaft 104 and the end-portion having a symmetry of orderN may be one piece. However, in some embodiments the shaft end-portionis a separate part, which is attached to the shaft 104, e.g. byclamping, gluing, press-fit, etc.

In the examples shown in FIGS. 8 to 11 the shaft end portion is orincludes a flexible (elastically deformable) part 151, which is attachedto the shaft 104 and thus rotates synchronously with the shaft 104. Theflexible part 151 of the shaft end-portion can be designed such that itcompensates for an axial play or axial tolerances of the shaft 104. Inthe embodiment shown in FIG. 8 the flexible part 151 is a flat spring,which is attached to the shaft 104 by an anchor element 150. In anunbent state the flat spring 151 is substantially plane, perpendicularto the rotation axis, and arranged in front of a front side of the shaft104. The rotation axis of the shaft 104 intersects with a longitudinalaxis of the flat spring. In the examples shown in FIGS. 8 and 9, thelongitudinal axis of the flat spring is parallel to the x-axis, andgenerally the longitudinal axis of the flat spring extends radially andintersects the rotation axis. In general FIG. 8 illustrates alongitudinal section along the z-axis and is essentially identical withthe examples shown in FIGS. 3 and 6, except that the shaft end-portionis composed of the flat spring 151, the mentioned anchor element 150 anda spacer 152, which is arranged between the sensor package 106 and theflat spring 152.

In one embodiment, the shaft exhibits an axial play of about +/−1 mm,and the flat spring 151 bends more or less in order to maintain amechanical contact with the sensor package 106 via the spacer 152. Theanchor element 150 may be configured to fix the flat spring 151 to therotating shaft 104 such that the flat spring is flexible along an axialdirection (z-direction) but comparably stiff in lateral (i.e. x-y-)directions. The flat spring 151 may be made of or include a softmagnetic material (e.g. spring steel), so that it takes over themagnetic function of the groove P shown in the previous embodiments(See. FIGS. 1 and 3). Alternatively, the flat spring 151 may be composedof a non-magnetic spring body (e.g. made of beryllium bronze or copperberyllium) and a soft magnetic element attached to the non-magneticspring body. The purpose of the flexible element is that, regardless ofsmall axial position changes of the shaft, the flexible element ensuresthat a soft magnetic part, which is not rotationally symmetric, is keptat a substantially constant axial distance to the sensor elements. Thisnot rotationally symmetric soft magnetic part may be the spring itself(as in FIGS. 8-11) or any other soft magnetic part attached to theflexible end of the spring (not shown). Since the axial spacing betweenthe soft magnetic part and the sensor elements is kept substantiallyconstant, axial play of the shaft may have less effects on the angularposition measurement and it should result in smaller errors in themeasured angular position value.

In the embodiment shown in FIG. 8, small spacer 152 is attached to theflat spring 151 at the rotation axis. Thus the spacer 152 can rotatearound the rotation axis synchronously with the shaft 104, whereby thespacer defines the mechanical contact point between shaft end-portionand sensor package 106 and the axial distance between the flexible shaftend-portion and the sensor package 106. The spacer 152 can be made ofe.g. Teflon (polytetrafluoroethylene, PTFE) or some other kind ofmaterial that ensures low friction between spacer 152 and the sensorpackage 106. Generally, the sensor packages 106 includes a moldcompound, which contains an abrasive filler. Thus, low friction may be atarget when choosing the material for the spacer. The spacer may bemagnetic or not; due to its rotational symmetry the spacer would notgive rise to any magnetic field that varies dependent on the angularposition.

The flexible part of the shaft end portion (e.g. the flat spring) mayhave various shapes. Two exemplary embodiments of the flat spring 151are shown in FIGS. 9a and 9b , both of which are a top view onto theflat spring corresponding to the sectional view of FIG. 8. In theembodiment shown in FIG. 9a the flat spring 151 has the shape of a smallstrip-like plate with a width w_(S) smaller than the sensor chip 110(width w_(C)) in the xy-plane, so that the flat spring 151 covers only apart of the sensor chip 110 (see top view of FIG. 9a ). In thealternative embodiment of FIG. 9b , the width w_(S) of the flat spring151 is greater than the sensor chip 110 and thus covers the whole sensorchip. A slot is formed in the center of the flat spring 151 along itslongitudinal axis (x-axis in FIG. 9b ) so that the rotation axis runsthrough the slot. The width w_(SL) of the slot may be smaller than thewidth w_(C) of the chip (as it is the case in the example of FIG. 9b ).In this embodiment the effect of the slot on the resulting magneticfield “seen” by the magnetic field sensor elements in the sensor chip110 is very similar to the effect of the groove P used in the examplesillustrated in FIGS. 3 and 6. Thus, in the present embodiment shown inFIGS. 8 and 9 the shaft end portion has a symmetry of order 2 (N=2,two-fold rotational symmetry) for angular position measurement withinthe range from 0° to 180°. In another embodiment the flat spring 151 hasa tapered geometry (i.e. its width being smaller at one and as comparedwith the other end) to achieve a symmetry of order 1 and allow for anangular position measurement within the range from 0° to 360°. Moreover,the strip-like plate 151 (i.e. the spring) in FIG. 9a may be wider andlonger and shifted in negative y-direction such that the upper edge ofthe spring is at y=0 and the spring 151 covers the part of the chip, forwhich y<0 (see dotted area in FIG. 9a ). If this spring is ferromagnetic(i.e. soft magnetic) and magnetized by a permanent magnet 102 fixed tothe sensor package or the PCB 108 or by a permanent magnet 102′ fixed tothe shaft, and if these magnets are rotationally symmetry with arotationally symmetric remanent magnetization, then the spring 151creates—at the position of the magnetic field elements—a magnetic fieldwith symmetry of order N=1, i.e. its magnetic field on the sensorelements allows for unambiguous measurement of angles in a range from 0°to 360°. The same is true for a wide strip in FIG. 9b , if the widthw_(SL) of the slot is wide enough (e.g. wider than w_(C)/2) and longenough (e.g. longer than the chip) and if the slot is shifted towards anegative y-direction such that its upper edge is aligned with thex-axis.

In the embodiment of FIG. 10, the flat spring 151 is bent by 90° anddirectly attached to the shaft 104 instead of using a separate anchorelement (such as, e.g., anchor element 150 in FIG. 8). For example, theportion of the flat spring 151 which is attached to the shaft 101 may beinserted in a groove, which extends axially in the circumferentialsurface of the shaft, and screwed, glued, welded or fixed otherwise tothe shaft. Generally, it is also possible to emboss the flat spring 152at the rotation axis instead of attaching a separate spacer. In thiscase the spacer is implemented as an embossment 152′ as shown in theexamples of FIGS. 10 and 11. In the example of FIG. 11, the flat springhas an S-shape and is attached to the front side of the shaft 104instead of to the circumferential surface as in the previous example ofFIG. 10. In this case an additional asymmetry (symmetry of order 1) isachieved by an asymmetric recess R in the front side of the shaft 104.The effect of the recess R on the resulting magnetic field “seen” by themagnetic field sensors is similar to the effect of the front side of theshaft being tilted as in the example of FIG. 1. As in the previousembodiments a permanent magnet 102 may be attached to the PCB 108.Additionally or alternatively, another permanent magnet 102′ may beattached to the shaft 104 (shown in dotted lines in FIG. 10).

In the embodiments shown herein, the sensor chip 110 is closer to thespacer 152 or the embossment 152′ than the leadframe 116, which ensuresa low distance (air gap) between shaft end-portion and magnetic fieldsensor element integrated in the sensor chip 106. Such an arrangementmay help to maximize the level of the magnetic field at the sensorelements and the signal output of the magnetic field sensor elements. Inother embodiments, however, the sensor chip may be reversed so that anexposed metal die-paddle is contacted by the spacer 152 or embossment152′ instead of the plastic portion (mold compound) of the sensorpackage. In this case the metal die-paddle protects the sensor package106 against abrasion caused by the rotating spacer or embossment. Evenif the die-paddle were not exposed (i.e. it is covered by mold compoundand therefore the spacer 152 or embossment 152′ is in contact with theabrasive mold compound) it would still protect the chip from the spring151. In an alternative embodiment, the magnetic field sensor package 106can be mounted to the bottom side of the PCB, while the shaft faces thefront side of the component board, and the spacer 152 is in contact withthe front side surface of the PCB or with a metal plate or similarstructure mounted to the front side of the PCB. In this embodiment thereis no friction between the rotating spacer and the abrasive moldcompound of the sensor package and the sensor package is protected fromthe rotating spacer 152 by the PCB in-between.

Generally, the permanent magnets can be mounted on the PCB 108 (seeFIGS. 10 and 11), the sensor package 106 (see FIG. 1), the shaft 104(see FIG. 6), the flat spring 151 (e.g. on its flexible end), or betweenshaft and spring, or even on the anchor element 150. The permanentmagnets can have various shapes such as the shape of a ring, pill,cylinder, cuboid with various directions of magnetization (purely axialor purely radial or combinations of axial and diametrical or axial andradial). Further embodiments may be created by combining various aspectsof the embodiments discussed above.

The following FIGS. 12 to 14 illustrate different arrangements (layouts)of the magnetic field sensor elements integrated in at least one sensorchip 110 (see, e.g. FIGS. 3, 6, and 8). Generally, at least fourmagnetic field sensor elements are used in the embodiments describedherein, although the embodiments shown in FIGS. 12 to 14 use eight orsixteen magnetic field sensor elements (wherein, in the latter case, tworespective magnetic field sensor elements are arranged closely adjacentto or adjoining each other and their sensor output is averaged, seeFIGS. 12 and 13). The sensor chip 110 defines a sensor plane which isarranged substantially perpendicular to the rotation axis (z-axis) andin which the magnetic field sensor elements are arranged. Generally, afirst and a second one of the magnetic field sensor elements are spacedapart from each other and are only sensitive to magnetic fieldcomponents in a first direction (e.g. x-direction) and a third and afourth one of the magnetic field sensor elements are also spaced apartfrom each other and are only sensitive to magnetic field components in asecond direction (e.g. y-direction). As mentioned above, thex-direction, the y-direction and the z-direction (defined by therotation axis) form a Cartesian coordinate system. Thus, the first andthe second directions are non-parallel and perpendicular to the rotationaxis. As mentioned above, the sensor elements used in the embodimentsdescribed herein are substantially insensitive to magnetic fieldcomponents perpendicular to the sensor plane (and thus parallel to therotation axis). In the embodiments described herein, the individualmagnetic field sensor elements may be arranged, for example, along acircle of radius r, which is concentric to the rotation axis. At (orvery close to) angular positions 0° (i.e. at P₁=(r, 0)), 90° (i.e. atP₂=(0, r)), 180° (i.e. at P₃=(−r, 0)), and 270° (i.e. at P₄=(0, −r))magnetic field sensor elements are provided, which are sensitive tomagnetic field components in the x- and y-direction but insensitive inz-direction (H_(X) sensor elements and H_(Y) sensor elements).

In the embodiments described herein, sensor elements 114 a, 114 b, 116a, 116 b, 118 a, 118 b, 120 a, 120 b are sensitive to magnetic fieldcomponents in y-direction and are thus referred to as H_(Y) sensorelements. Similarly, sensor elements 115 a, 115 b, 117 a, 117 b, 119 a,119 b, 121 a, 121 b are sensitive to magnetic field components inx-direction and are thus referred to as H_(X) sensor elements.Theoretically, the magnetic field sensor elements (e.g. sensor elements114 a, 114 b, 115 a, and 115 b in FIG. 12) should be provided at thevery same spot (at locations P₁, P₂, P₃, P₄) on the circle with radiusr, which is difficult to implement as the magnetic field sensor elementswould have to be stacked. The example of FIG. 12 illustrates oneimplementation, according to which two H_(X) sensor elements and twoH_(Y) sensor elements are arranged symmetrically and closely adjacent tothe angular positions P₁, P₂, P₃, and P₄ on the circle of radius r. InFIG. 12, pairs of H_(Y) sensor elements 114 a, 114 b, 116 a, 116 b, 118a, 118 b, and 120 a, 120 b are aligned with the x-axis (sensor elements114 a, 114 b, 118 a, 118 b) or with an axis parallel to the x-axis(sensor elements 116 a, 116 b, 120 a, 120 b), and each pair is arrangedsymmetric to the respective position P₁, P₂, P₃, and P₄. The magneticfield components H_(X) and H_(Y) for a specific position P₁, P₂, P₃, andP₄ can be obtained by averaging the output signals of the sensorelements of each pair. For example, the output signals of H_(Y) sensorelements 114 a and 114 b are averaged to obtain a measurement of they-component H_(Y) of the magnetic field at position P₁. Analogously, theoutput signals of H_(X) sensor elements 121 a and 121 b are averaged toobtain a measurement of the x-component H_(X) of the magnetic field atposition P₄. In essence, the sensor elements 114 a and 114 b can beregarded as one single (but distributed) H_(Y) sensor element at thelocation P1. Analogously, the sensor elements 117 a and 117 b can beregarded as one single H_(X) sensor element at the location P2, etc. Inaccordance with another implementation the sensor elements are not“split” into pairs, but the H_(X) and H_(Y) sensor elements are providedso close to the desired positions P₁, P₂, P₃, and P₄, that the deviationbetween actual sensor position and desired position (P₁, P₂, P₃, and P₄)is negligible. The example of FIG. 13 is essentially the same as theprevious example of FIG. 12. In FIG. 13, however, the positions P₁′,P₂′, P₃′, and P₄′, at which the magnetic field components H_(X) andH_(Y) are measured, are shifted by 45° as compared with the positionsP₁, P₂, P₃, and P₄ shown in FIG. 12. Generally, the spacing betweendifferent sensing locations on the sensor plane is at least as high as(or at least twice as high as) the largest dimension of a singlemagnetic field sensor element.

In the example of FIG. 14 one magnetic field sensor element is providedon the circle with radius r at each one of the following angularpositions on the circle: 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°.One H_(Y) sensor element is provided on the circle an angle of 0°, 135°,180°, and 315° (H_(Y) sensor elements 114 b′, 116 a′, 114 a′, 116 b′).One H_(X) sensor element is provided on the circle an angle of 45°, 90°,225°, and 270° (H_(X) sensor elements 117 b′, 115 b′, 117 a′, 115 a′).As only a single magnetic field sensor element is provided at eachposition, the magnetic field sensor elements can be placed exactly onthe desired position. For all three embodiments shown in FIGS. 12 to 14,one important aspect is that only H_(X) and H_(Y) sensor elements areused which have a sensitivity in a direction perpendicular to therotation axis (z-direction) and which are not affected by magnetic fieldcomponents in z-direction. Furthermore, the individual magnetic fieldsensor elements are aligned such that they measure magnetic fieldcomponents in the x-direction and the y-direction and not in anarbitrary radial direction (e.g. along the 45° line through points P₁′and P₃′ in FIG. 13). In practice, it may be difficult to exactly align asensor element in an arbitrary radial direction, whereas it is simplerto align it x- and y-directions, because x- and y-directions areparallel to the edges of the chip and this is the usual grid along whichelectronic devices are aligned in common microelectronic technologies.It should be noted, however, that the output signals of diametricallyarranged pairs of magnetic field sensor elements 114 a′ 114 b′, 115 a′115 b′, 116 a′ 116 b′, and 117 a′ 117 b′ are not averaged, whereas inthe examples of FIGS. 12 and 13 the output signals of correspondingpairs of magnetic field sensor elements 114 a, 114 b, 115 a, 115 b, etc.are averaged to obtain one average output signal.

Moreover, it should be clear that other alignments of the sensorelements like radial or azimuthal orientation are also possible, becauseby straightforward transformation of coordinate systems one can expressa field in a first direction as a combination of field components in atleast two non-parallel other directions. It is also not necessary toplace all sensor elements on a circle with center on the rotation axisas shown in the present embodiment (the center point may be defined byprojecting the rotation axis onto the sensor plane). In principle, thesensor elements may be placed on regular or even irregular grids in thex-y-plane to sample the field components H_(X)(x,y) or H_(Y)(x,y) orH_(R)(x,y) (i.e. the radial component) or H_(PSI) (x,y) (i.e. theazimuthal component) and reconstruct the functions H_(X)(x,y) orH_(Y)(x,y) or H_(R)(x,y) or H_(PSI)(x,y) by interpolation orapproximation like least square error fits or similar mathematicalmethods.

The following description deals with the algorithms, which may be usedto derive an angular position of the shaft from the output signals ofthe magnetic field sensor elements arranged according to the layoutsshown in FIGS. 12 to 14. There are numerous possibilities to determinethe angular position, and only a few examples (for a two-fold symmetry,i.e. N=2) are discussed as representatives of a wide class ofalgorithms. Generally, the embodiments described herein may includecircuitry, which is coupled to the (at least four) magnetic field sensorelements integrated in the sensor chip 110 and which is configured tocalculate an angular position of the shaft by combining the outputsignals of the magnetic field sensor elements. One common aspect ofthese approaches is that two or more output signals of the magneticfield sensor components are combined in such a manner that two signalsare obtained, which (1) have a defined amplitude relation (e.g. equalamplitudes), (2) have a defined phase lag (e.g. 90°), and (3) areinsensitive against homogeneous disturbing magnetic fields. The lastproperty may be achieved by using a differential measurement. That is,the output signals of a pair of H_(X) sensor elements (or a pair ofH_(Y) sensor elements) are subtracted to obtain a differential signal inwhich the signal components resulting from a homogenous disturbing fieldare cancelled out. In the example of FIG. 14 differential signals can beobtained from a pair of sensor elements, which are located spaced apartfrom each other at two diametrically opposing positions on the circlewith radius r (i.e. 114 a′ and 114 b′, 115 a′ and 115 b′, 116 a′ and 116b′, etc.). Instead of a subtraction, a weighted sum of output signals ofdifferent magnetic field sensor elements may be used (wherein negativeweight factors are possible). As mentioned, the magnetic field sensorelements need not necessarily be arranged along a circle. Generally,magnetic field components in the first direction (e.g. x-direction) aresensed at least at a first and a second location in the sensor plane(e.g. locations P1 and P3 in the example of FIG. 12). Additionally,magnetic field components in the second direction (e.g. y-direction) aresensed at at least a third and a fourth location in the sensor plane(e.g. locations P1 and P4 in the example of FIG. 12). The sought angularposition of the shaft with respect to its rotation axis can then becalculated based on the difference of the magnetic field components atthe first and the second location and on the difference of the magneticfield components at the third and the fourth location. The mentioneddifferences are used to implement the above-mentioned differentialmeasurement. As such, the angular position sensor system may be regardedas a kind of gradiometer.

Referring to the example of FIG. 12, a signal S_(COS), which isproportional to the cosine of twice the angular shaft position φ(S_(COS)˜cos(2φ)), is obtained by calculating(H_(X)(0°)−H_(X)(180°))−(H_(Y)(90°)−H_(Y)(270°)). A signal S_(SIN),which is proportional to the sine of twice the angular shaft position φ(S_(SIN)˜sin(2φ)), is obtained by calculating H_(Y)(0°)−H_(Y)(180°).Another signal S_(SIN)′, which is proportional to the sine of twice theangular shaft position φ, is obtained by calculatingH_(X)(90°)−H_(X)(270°). The cosine signal S_(COS) and any of the twosine signals S_(SIN), S_(SIN)′ may be used to calculate the tangentfunction tan(2φ) (ratio S_(SIN)/S_(COS) or S_(SIN)′/S_(COS)) of theangular position 2φ, from which the sought angular position can becalculated as the arc tangent of, e.g., S_(SIN)/S_(COS). For example,the CORDIC algorithm may be used to implement the arc tangent function.When using the layout as shown in FIG. 12, the magnetic field componentH_(Y)(0°) is obtained by averaging the output signals of magnetic fieldsensor elements 114 a and 114 b. The same is done for the H_(X)components and other positions of magnetic field sensor elements. Thefactor 2 in the expressions cos(2φ), sin(2φ), and tan(2φ) are due to thementioned two-fold symmetry of the set-up and the angular position canonly be unambiguously determined within a range from 00 to 180°, whichmay be sufficient when the on-axis angular position sensor is used tocontrol certain types of BLDC motors.

When using the layout in accordance with the example of FIG. 13, thesignal S₁ and S₂ may be calculated as

S ₁=(H _(X)(45°)−H _(X)(225°))+(H _(Y)(315°)−H _(Y)(135°)), and

S ₂=(H _(X)(135°)−H _(X)(315°))+(H _(Y)(45°)−H _(Y)(225°)),

wherein the sum of S₁ and S₂ equals A·sin(2φ) and the difference S₁−S₂equals A·cos(2φ). Similar as in the previous example the angularposition p may be derived as ½·arctan((S₁+S₂)/(S₁−S₂)).

When using the layout in accordance with the example of FIG. 14, a sinesignal S_(SIN) and a cosine signal S_(COS) may be calculated as

S _(SIN)=(H _(X)(270°)−H _(X)(90°))−(H _(Y)(0°)−H _(Y)(180°)), and

S _(COS)=√{square root over (2)}·SC−S _(SIN),

wherein SC=(H_(X)(225°)−H_(X)(45°))−(H_(Y)(315°)−H_(Y)(135°)). Asmentioned, the examples discussed above relate to a two-fold symmetry ofthe shaft end-portion. It is noted that similar expressions can be foundfor signals suitable in measurement set-ups with a symmetry of order oneor order three or higher. In the equations above, the sensing locationsP1, P1′, P2, P2′, P3, P3′, P4, and P4′ are represented by acorresponding angle (0° denotes P1, 45° denoted P1′, 90° denoted P2,135° denotes P2′, 180° denoted P3, 225° denoted P3′, 270° denoted P4,and 315° denotes P4′). Together with the radius r the angleunambiguously defines the respective sensing locations P1, P1′, P2, P2′,P3, P3′, P4, and P4′.

As mentioned above, a two-fold symmetry of the shaft end portion may besufficient in at least some applications, in which the rotational motionBLDC motor is to be controlled. FIGS. 15 and 16 illustrate two differentexamples of how an on-axis angular position sensor can be arranged in abrushless DC (BLCD) motor assembly. FIG. 15 is a sectional view of aportion of a BLDC motor, wherein the sectional plane runs through therotation axis of the shaft 104. It is noted, however, that FIG. 15 isnot in scale and has to be regarded as a schematic sketch. Also shown inFIG. 15 are the permanent magnets (PM) 217 and 218, which are directlyor indirectly attached to the shaft 114 along its circumference. Theshaft 104 may be supported by at least two bearings, wherein bearing 214is a ball bearing in the depicted example. However, other types ofbearings may also be used dependent on the actual implementation. Shaft104 and the PMs attached to the shaft form the rotor (armature) of themotor. The stator 210 of the motor includes coils 211 and 212, which aresupplied with current to control the angular motion of the motor.

As in the examples described above, the end-portion of the shaft 104 hasa recess P similar to the examples shown in FIGS. 3 and 4, wherein apermanent magnet 102′ is attached to the shaft 104 similar to theexample discussed above with reference to FIG. 6. To realize a compactdesign, the magnetic field sensors (represented in FIG. 15 by sensorchip 106) used to form an on-axis angular position sensor are arrangedon PCB 108 together with the power electronics used to generate theoperating current supplied to the stator coils 211, 212.

As can be seen from FIG. 15, the PCB directly faces the front side ofthe shaft 104 (i.e. the shaft end portion), and the magnetic fieldsensor elements are, in the present example, arranged on the PCB 108symmetrically to the rotation axis as shown in FIGS. 12 to 14. Theelectrical contacts C (soldering contacts, pins) of the stator coils211, 212 protrude from the stator towards the PCB 108, which allows fordirect electrical connection between the coils 211, 212 and the PCB 108.The PCB 108 may be supported at an inner surface of a motor housing Hthat faces the stator 210 of the motor.

The example of FIG. 16 is essentially the previous example of FIG. 15,except that the permanent magnet 102′ attached to the shaft end portionis omitted and a permanent magnet 102 is instead arranged on the backside of the PCB 108 similar to the example shown in FIG. 3. It isunderstood that any other angular position sensor arrangement may beused in a BLDC motor assembly as shown in the FIGS. 15 and 16.

The PCB 108 carries the sensor chip 106 as well as power devices 215,216, which provide the (controlled) load current supplied to the statorcoils 211, 212. The distance between the magnetic field sensor elements(in the sensor chip 106) and the power devices 215, 216, or between themagnetic field sensor elements and load current lines on the PCB 108 isoften comparably small (e.g. less than 10 mm or 20 mm) so that themagnetic field generated by the current traces or power devices isstrongly inhomogeneous at the locations of the magnetic sensor elements.Therefore, a common gradiometer is not enough to eliminate the effect ofthe disturbing magnetic fields caused by the load current lines and thepower devices. However, the load current lines and the power devices are(at least approximately) located at the same z-position (i.e. the sameaxial position) as the magnetic field sensor elements (and therefore themagnetic field lines intersect the sensor elements almost vertically assketched in FIG. 2). The angular position measurement becomes veryrobust against these disturbances if the magnetic field sensor elementsdoes not respond to those magnetic field components in z-direction.Therefore, the embodiments described herein include gradiometer set-upsthat are sensitive to magnetic field components in x- or y-directions.

Although the invention has been illustrated and described with respectto one or more implementations, alterations and/or modifications may bemade to the illustrated examples without departing from the spirit andscope of the appended claims. In particular regard to the variousfunctions performed by the above described components or structures(units, assemblies, devices, circuits, systems, etc.), the terms(including a reference to a “means”) used to describe such componentsare intended to correspond—unless otherwise indicated—to any componentor structure, which performs the specified function of the describedcomponent (e.g., that is functionally equivalent), even if notstructurally equivalent to the disclosed structure, which performs thefunction in the herein illustrated exemplary implementations of theinvention.

In addition, while a particular feature of the invention may have beendisclosed with respect to only one of several implementations, suchfeature may be combined with one or more other features of the otherimplementations as may be desired and advantageous for any given orparticular application. Furthermore, to the extent that the terms“including”, “includes”, “having”, “has”, “with”, or variants thereofare used in either the detailed description and the claims, such termsare intended to be inclusive in a manner similar to the term“comprising”

We claim:
 1. A magnetic angular position sensor system comprising: ashaft rotatable around a rotation axis, the shaft having a soft magneticshaft end portion; a sensor chip spaced apart from the shaft end portionin an axial direction and defining a sensor plane, which issubstantially perpendicular to the rotation axis; at least four magneticfield sensor elements integrated in the sensor chip, wherein two of themagnetic field sensor elements are spaced apart from each other and areonly sensitive to magnetic field components in a first direction andwherein two of the magnetic field sensor elements are spaced apart fromeach other and are only sensitive to magnetic field components in asecond direction; the first and the second direction being mutuallynon-parallel and the first and the second direction being perpendicularto the rotation axis; a magnetic field source that magnetizes the shaftend portion, wherein the shaft end portion is formed such that amagnetic field in the sensor plane, which is caused by the magneticfield source, is rotationally symmetric with order N, N being a finiteinteger number ≧1; and circuitry coupled to the at least four magneticfield sensor elements and configured to calculate an angular position ofthe shaft by combining output signals of the at least four magneticfield sensor elements.
 2. The magnetic angular position sensor systemaccording to claim 1, wherein the at least four magnetic field sensorelements are arranged in the sensor plane around a center point definedby a projection of the rotation axis onto the sensor plane.
 3. Themagnetic angular position sensor system according to claim 1, whereinthe a magnetic field source comprises at least one permanent magnet. 4.The magnetic angular position sensor system according to claim 3,wherein the sensor chip is arranged in a chip package, which is attachedto a circuit board, and wherein the at least one permanent magnet isattached to the chip package or the circuit board or the shaft endportion.
 5. The magnetic angular position sensor system according toclaim 1, wherein the a magnetic field source comprises a first permanentmagnet and at least a second permanent magnet; wherein the sensor chipis arranged in a chip package, which is attached to a circuit board, andwherein the first permanent magnet is attached to the chip package orthe circuit board and the second permanent magnet is attached to theshaft end portion.
 6. The magnetic angular position sensor systemaccording to claim 1, wherein the shaft end portion has at least onegroove or at least one protrusion arranged in or on its front side, thegroove or the protrusion is shaped such that the shaft end portion has arotational symmetry of order N, N being a finite integer number.
 7. Themagnetic angular position sensor system according to claim 1, whereinthe shaft end portion includes a spring rotating synchronously with theshaft, the spring being essentially resilient in an axial direction ofthe rotation axis.
 8. The magnetic angular position sensor systemaccording to claim 7, wherein the spring is substantially non-resilientin directions perpendicular to the rotation axis.
 9. The magneticangular position sensor system according to claim 7, wherein the springis arranged between the sensor chip and a front side of the shaft endportion, and wherein the spring is supported on a chip package of thesensor chip via a spacer.
 10. The magnetic angular position sensorsystem according to claim 9, wherein the spacer is attached to thespring or the spacer is formed by an embossment of the spring, andwherein the spring is configured to exert an axial force on the chippackage.
 11. The magnetic angular position sensor system according toclaim 1, wherein the at least four sensor elements are symmetricallydistributed along a circle, which is coaxial with the rotation axis. 12.A magnetic angular position sensor system comprising: a shaft rotatablearound a rotation axis, the shaft having a soft magnetic shaft endportion; a sensor chip spaced apart from the shaft end portion in anaxial direction and defining a sensor plane, which is substantiallyperpendicular to the rotation axis; at least four magnetic field sensorelements integrated in the sensor chip, a first and a second magneticfield sensor element of the at least four magnetic field sensor elementsbeing spaced apart from each other and sensitive to magnetic fieldcomponents in a first direction, a third and a fourth magnetic fieldsensor element of the at least four magnetic field sensor elements beingpaced apart from each other and sensitive to magnetic field componentsin a second direction, wherein the first and the second directions aremutually non-parallel and the first and the second directions areperpendicular to the rotation axis; a magnetic field source thatmagnetizes the shaft end portion, wherein the shaft end portion isformed such that a magnetic field in the sensor plane, which is causedby the magnetic field source, is rotationally symmetric with order N, Nbeing a finite integer number ≧1; and a signal processing circuitcoupled to the at least four magnetic field sensor elements andconfigured to: calculate a first signal representing the difference ofthe magnetic field components sensed by the first and the secondmagnetic field sensor elements; calculate a second signal representingthe difference of the magnetic field components sensed by the third andthe fourth magnetic field sensor elements; and calculate an angularposition of the shaft by combining at least the first and the secondsignal.
 13. The magnetic angular position sensor system according toclaim 12, wherein the at least four sensor elements are symmetricallydistributed along a circle, which is coaxial with the rotation axis. 14.The magnetic angular position sensor system according to claim 13,wherein a first portion of the magnetic field sensor elements arearranged along a first circle, and wherein a second portion of themagnetic field sensor elements are arranged along a second circle, thefirst and the second circle being coaxial with the rotation axis andhave different radii.
 15. The magnetic angular position sensor systemaccording to claim 12, wherein the first and the second magnetic fieldsensor elements are arranged diametrical with respect to the rotationaxis; and/or wherein the third and the fourth magnetic field sensorelements are arranged diametrical with respect to the rotation axis. 16.The magnetic angular position sensor system according to claim 12,wherein a first edge of the sensor chip in the first surface is alignedparallel to the first direction, and a second edge of the sensor chip inthe first surface is aligned parallel to the second direction.
 17. Anelectric motor assembly comprising: a stator including at least onestator coil; a rotor including at least a shaft having a front side anda soft-magnetic shaft end portion; a printed circuit board (PCB)arranged such that it faces the front side of the shaft, at least onesensor chip attached to the PCB and spaced apart from the shaft endportion; at least four magnetic field sensor elements arranged in the atleast one sensor chip, wherein two of the magnetic field sensor elementsare spaced apart from each other and only sensitive to magnetic fieldcomponents in a first direction and two of the magnetic field sensorelements are spaced apart from each other and only sensitive to magneticfield components in a second direction; the first and the seconddirection being substantially mutually non-parallel and the first andthe second direction being perpendicular to the rotation axis a magneticfield source that magnetizes the shaft end portion, wherein the shaftend portion is formed such that the magnetic field components in thefirst and the second direction are rotationally symmetric with order N,N being a finite integer number ≧1; and an evaluation circuit coupled tothe at least four magnetic field sensor elements and configured tocalculate an angular position of the shaft by combining at least fouroutput signals of the at least four magnetic field sensor elements;power electronic circuitry arranged on the PCB and coupled to the statorcoils and configured to supply an operating current to the stator coils.18. A method for measuring an angular position of a shaft that includesa soft magnetic shaft end portion; the method comprising: magnetizingthe shaft end portion, wherein the shaft end portion is formed such thatthe magnetic field components in a first and a second direction in asensor plane, which is substantially perpendicular to a rotation axis ofthe shaft, are rotationally symmetric with order N, N being a finiteinteger number ≧1; sensing magnetic field components in the firstdirection at at least a first and a second location in the sensor plane,the second location being different from the first location; sensingmagnetic field components in the second direction at at least a thirdand a fourth location in the sensor plane; the fourth location beingdifferent from the third location; calculating an angular position ofthe shaft with respect to its rotation axis based on the difference ofthe magnetic field components at the first and the second location andon the difference of the magnetic field components at the third and thefourth location.
 19. The method of claim 18, wherein sensing magneticfield components comprises: providing a first sensor signal using atleast a first magnetic field sensor element configured to sense magneticfield components in the first direction at the first location; providinga second sensor signal using at least a second magnetic field sensorelement configured to sense magnetic field components in the firstdirection at the second location; providing a third sensor signal usingat least a third magnetic field sensor element configured to sensemagnetic field components in the second direction at the third location;and providing a fourth sensor signal using at least a fourth magneticfield sensor element configured to sense magnetic field components inthe second direction at the fourth location.
 20. The method of claim 19,wherein calculating an angular position of the shaft comprises:calculating a fifth signal representing the difference between the firstand the second sensor signal; calculating a sixth signal representingthe difference between the third and the fourth sensor signal; and andcalculating an angular position based on the fifth and the sixth signal.